1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine in which a selective catalytic reduction (SCR) catalyst is arranged in an exhaust passage.
2. Description of the Related Art
As an internal combustion engine operated by the use of a mixture with an air fuel ratio (i.e., a lean air fuel ratio) higher than a theoretical air fuel ratio, there has been known one in which a selective catalytic reduction catalyst (SCR catalyst) is arranged in an exhaust passage. In such an internal combustion engine, as a method of detecting an abnormality of the SCR catalyst, there has been known a method of determining the degree of deterioration of the SCR catalyst based on an amount of moisture which can be adsorbed by the SCR catalyst, by paying attention to the fact that there is a correlation between the amount of moisture which can be adsorbed by the SCR catalyst and the degree of deterioration of the SCR catalyst. In addition, as a method of specifying the amount of moisture which can be adsorbed by the SCR catalyst, there has been proposed one that captures or detects a phenomenon in which the temperature of an exhaust gas is decreased due to the absorption of moisture by the heat of the exhaust gas at the time when the moisture is desorbed from the selective catalytic reduction catalyst (for example, see a first patent literature).